As an optical recording medium (hereinafter referred to as “optical disk”) having an information recording layer formed on a light incident side surface and a cover layer being a transparent resin layer covering the information recording layer, an optical disk for CD (hereinafter referred to as CD optical disk or “CD”) having a cover layer thickness for information recording layer (hereinafter referred to as “cover thickness”) of 1.2 mm, or an optical disk for DVD (hereinafter referred to as DVD optical disk or “DVD”) having a cover thickness for information recording layer of 0.6 mm and the like are widely used. Meanwhile, as an optical head device to be used for writing and/or reading (hereinafter referred to as “writing/reading” of an information to/from a CD, one having a laser diode of wavelength λ3 (hereinafter referred to as “wavelength λ3 of CD”) in a 790 nm band as a light source and an objective lens of NA (numerical aperture) 0.45 to 0.50, has been known. Further, as an optical head device to be used for writing/reading to/from a DVD, one having a laser diode of wavelength λ2 (hereinafter referred to as “wavelength λ2 of DVD”) in a 660 nm band as a light source and an objective lens of NA 0.6 to 0.65, is employed.
Further, in recent years, in order to improve recording density of an optical disk, an optical disk having a cover thickness of 0.1 mm (hereinafter referred to as BD optical disk or “BD”) has been developed. As an optical head device to be used for writing/reading such a BD, one having a blue laser diode of wavelength λ1 (hereinafter referred to as “wavelength λ1 of BD”) in a 405 nm wavelength band as a light source and an objective lens of NA 0.85, is employed. However, when writing/reading to/from a DVD or a CD is carried out using an objective lens optimally designed to make wavefront aberration substantially zero for BD at the wavelength λ1 (hereinafter referred to as “objective lens for BD”), a large spherical aberration due to the difference of cover thicknesses of optical disks, is generated. As a result, convergence of incident light on an information recording layer is deteriorated, which prevents good writing/reading. Under these circumstances, development of small sized three-wavelength compatible optical head device has been investigated, which can write/read three types of optical disks having different cover thicknesses using single objective lens. By the way, as one of the optical head devices to realize such a three-wavelength compatible optical head device, an optical head device has been proposed (for example, JP-A-2004-71134), which can write/read a DVD using an objective lens for BD and a Fresnel lens.
The optical head device employs as a Fresnel lens, a hologram grating having a step-shaped cross-section formed in a region corresponding to NA for DVD. The Fresnel lens is formed to have a (Fresnel lens) shape exhibiting a concave lens function at the wavelength λ2 for DVD, in which each unit step height of the step-shaped cross-section corresponds to an optical path difference of about one wavelength for light of wavelength λ1 of BD, and the Fresnel lens is to be used integrally with an objective lens for BD. By this construction, an optical head device which can write/read BDs and DVDs is constituted. However, since there is no aberration correction function for CDs, it is difficult to write/read three types of optical disks.
Further, an optical head device which reduces a spherical aberration generated due to the difference between cover thicknesses of CD, DVD and BD by a phase corrector, has been proposed (for example, JP-A-2003-207714).
The optical head device has a phase correction element for DVD which is a phase correction surface having a step-shaped cross-section, formed in a region corresponding to NA for DVD, and a phase correction element for CD which is a phase correction surface having a step-shaped cross-section formed in a region corresponding to NA for CD, and they are used integrally with an objective lens for BD.
Each unit step height of a step-shaped cross-section of a phase correction surface for DVD, is made to be a step height providing an optical path difference of substantially an integer times of wavelength λ1 of BD and wavelength λ3 of CD, so as to exhibit spherical aberration correction function only for wavelength λ2 for DVD. Further, each unit step height of the step-shaped cross-section of the phase correction surface for CD, is made to be a step height providing an optical path difference of substantially an integer times of wavelength λ1 of BD and wavelength λ2 of DVD, so as to exhibit spherical aberration correction function only for wavelength λ3 of CD.
However, in order to exhibit desired wavelength selectivity in each phase correction element, a glass material having a special refractive-index-wavelength-dispersion is required, and accurate fabrication of a plurality of deep step height is required, and thus, it is difficult to stably obtain wavelength-selective aberration correction function. Further, since the phase correction element corrects only spherical aberrations, the element does not show concave lens function for expanding the distance (hereinafter referred to as “working distance”) between the objective lens and an optical disk. Accordingly, in a case where the objective lens for BD is used integrally with a phase correction element for CD, working distance for CD becomes at most 0.3 mm, and it is difficult to stably write/read an optical disk without contact of the optical disk and the objective lens when the optical disk is rotating.
Here, a concave lens function can be exhibited by making the phase correction surface of the phase correction element for CD to have a Fresnel lens shape shown in JP-A-2004-71134. However, in this case, step height of the concave-convex portion and the number of annular rings are increased, which causes diffraction light of high diffraction order due to wall surfaces of the steps at the wavelengths λ1 of BD and λ2 of DVD, and thus, efficiency of transmission wavefront corresponding to desired concave lens function is reduced, such being problematic.
As means for correcting such a spherical aberration generated due to the difference of cover thicknesses of e.g. optical disks, an optical head device employing an optical modulation element corresponding to the liquid crystal element, has also been proposed (for example, JP-A-9-230300). FIG. 16 shows a lateral cross-sectional view of the optical modulation element.
The optical modulation element 100 comprises two transparent substrates 110 and 120 substantially in parallel with each other and a liquid crystal layer 130 sandwiched between them, and on a liquid crystal side surface of the transparent substrate 110, a Fresnel lens-shaped concave-convex portion 140 having a concentric blaze shape is formed. Further, on liquid crystal side surfaces of each of the transparent substrates 110 and 120, an electrode 150 and an alignment film 160 are formed. Further, a liquid crystal layer 130 has an alignment direction substantially in parallel with the transparent substrates at a time of no electric field application, and the alignment direction is substantially perpendicular to the transparent substrates at a time of electric field application.
Here, by constituting a construction in which any one of ordinary refractive index no or extraordinary refractive index ne of the liquid crystal layer 130 approximately equals to the refractive index nF of the concave-convex portion 140 of the transparent substrate having a blaze shape, a refractive index difference Δn between the liquid crystal layer 130 and the concave-convex portion 140 changes from Δn(=ne−no) to zero for extraordinarily polarized incident light at a time of no electric field application and at a time of electric field application. Accordingly, by making the depth of the concave-convex portion 140 to be Δn×(depth of concave-convex portion)=(wavelength of light in vacuum) and by making the refractive index nF of the concave-convex portion 140 substantially equal to ne, for extraordinarily polarized incident light, the optical modulation element 100 functions as a liquid crystal lens element, whose off-state having no concave lens function at a time of no voltage application and whose on-state exhibiting concave lens function at a time of voltage application for extraordinarily polarized incident light, are switchable.
When the optical modulation element 100 is integrated with an objective lens for BD and employed in an optical head device, and the optical modulation element 100 is made to be in on-state only at a time of writing/reading a CD, a spherical aberration generated due to the difference of cover thicknesses of optical disks can be corrected and a concave lens function of expanding a working distance to be at least 0.3 mm is exhibited. Meanwhile, at a time of writing/reading a BD or a DVD, high transmittance can be obtained by making the optical modulation element 100 to be in off-state.
However, when ordinarily polarized light is incident into the optical modulation element 100 shown in FIG. 16, transmission wavefront changes according to the refractive index difference Δn between the liquid crystal layer 130 and the concave-convex portion 140 regardless of the presence of applied voltage. Particularly, at a time of writing/reading a BD or a DVD, both of ordinarily polarized light and extraordinarily polarized light are incident into the optical modulation element 100, which deteriorates transmission wavefront aberration to cause a problem that writing/reading is prevented.
Further, heretofore, commonly used DVD optical disk has a single layer information recording layer and has a cover thickness of 0.6 mm (hereinafter referred to as “single layer DVD optical disk”). However, in recent years, in order to increase information amount in each optical disk, a (read only or readable and writable) optical disk having two information recording layers (hereinafter referred also to as “double layer DVD optical disk”) has been developed, and in the double layer optical disk, information recording layers are formed at positions corresponding to light incident side cover thicknesses of 0.57 mm and 0.63 mm.
Also with respect to BD optical disk, besides a BD optical disk having a single information recording layer and having a cover thickness of 0.1 mm (hereinafter referred to as “single layer BD optical disk”), a double layer optical disk (hereinafter referred to as “double layer BD optical disk”) has been developed to increase information amount in each optical disk, in which information recording layers are formed at positions corresponding to light incident side cover thicknesses of 0.100 mm and 0.075 mm.
Thus, in a case of employing an optical head device having an objective lens optimally designed to make zero aberration for a single layer optical disk (namely, a singe layer DVD optical disk or a single layer BD optical disk), to write/read to/from a double layer optical disk (namely, a double layer DVD optical disk or a double is layer BD optical disk), if their cover thicknesses are different, a spherical aberration is generated according to the difference in cover thicknesses, to deteriorate convergence of incident light into an information recording layer. Particularly, in a case of double layer optical disk of writable type, deterioration of convergent causes lowering of converging power density at a time of writing, which causes writing error, such being problematic.
To cope with this problem, as means for correcting a spherical aberration generated due to the difference of cover thicknesses of e.g. double layer optical disks, for example, an optical head device having a wavefront aberration correction means described in JP-A-10-269611 has been proposed. In this optical head device, as described in FIG. 2 of JP-A-10-269611, a segment liquid crystal panel is employed for correcting spherical aberration component generated according to the distance between recording layers of a multilayer disk.
However, in a case of correcting only spherical aberration component, if the liquid crystal panel is employed as it is disposed separately from an objective lens, misalignment between these elements occurs at a time of tracking operation of the objective lens, which causes a problem of comma aberration. In order to avoid such a problem, for example, it is considered to employ the liquid crystal panel integrally with the objective lens, but in such a construction, there has been a problem that load of an actuator for operating the objective lens increases and a voltage application system for the liquid crystal panel becomes complicated.
Further, in the same manner, for the purpose of correcting a spherical aberration generated due to the difference of cover thicknesses of e.g. double layer optical disks, an aberration correction device described in JP-A-2004-103058 has been proposed.
In this aberration correction device, as described in FIG. 2 of JP-A-2004-103058, an aberration correction unit is employed which comprises a hologram liquid crystal cell for rough adjustment which corrects large spherical aberration corresponding to the distance between recording layers of a multilayer optical disk, and a segment liquid crystal cell for fine adjustment which corrects spherical aberration corresponding to correlation error of cover layers. Here, the hologram liquid crystal cell constitutes a liquid crystal Fresnel lens comprising a glass substrate, another glass substrate having a saw-toothed-shaped cross-section, a liquid crystal sealed between these glass substrates and having a shape of diffraction grating whose cross-section has a saw-tooth-shaped blaze hologram shape, and transparent electrodes disposed on both sides of the liquid crystal for applying voltage to the liquid crystal. The electrodes are each made of non-divided uniform transparent conductive material.
However, in the case of this hologram liquid crystal cell, since the transparent electrode is formed on a surface of the glass substrate whose surface is fabricated into a saw-tooth shape, the transparent electrode tends to be broken, and thus, it has been difficult to produce stable and low-resistant transparent electrodes.
Further, in recent years, in order to improve recording density of an optical disk, an optical disk (hereinafter referred to as HDDVD optical disk or simply as “HD”) having the same cover thickness as DVD 0.6 mm, has been developed, which uses an optical head device having a blue laser diode of 405 nm wavelength band and an objective lens having a NA of 0.65. However, since HDDVD and BD have different cover thicknesses, there has been a problem that writing/reading of HDDVD using an objective lens for BD or writing/reading of BD using an objective lens for HDDVD is not possible.